1. Field of the Invention
The present invention relates generally to the production of cementitious compositions, and more particularly the production of compounds for use in filling and coating joints between adjacent gypsum wallboard sheets, for repairing imperfections in building materials, and for texturizing.
2. Description of Related Technology
One of the most common methods today of constructing interior walls includes the use of inorganic wallboard panels or sheets such as gypsum wallboard, often referred to simply as "wallboard" or "drywall." The use of wallboard, as opposed to conventional wet plaster methods, is often desirable because the installation of wallboard is ordinarily less costly and faster than that for conventional plaster. Wallboard is conventionally produced by enclosing a core of an aqueous slurry of gypsum and other additives, materials between two large sheets of paper. After the gypsum slurry has set and dried, the sheet is cut into standard sizes. Gypsum wallboard is described, for example, in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, 1970, Vol. 21, pages 621-24, the disclosure of which is hereby incorporated herein by reference.
A wall is generally made by securing, e.g., with screws and/or nails, the wallboard to a support structure, for example vertically or horizontally oriented pieces of wood. Because wallboard is typically supplied in standard-sized sheets or panels, when forming a wall from the sheets, there will generally be a number of joints between adjacent sheets. In most wallboard construction, it is necessary to conceal these joints between adjacent panels so that the wall will have a smooth, monolithic finish similar to that obtained with conventional wet plaster methods. It is also typically necessary to conceal the screws and/or nails used to secure the wallboard panels to framing, the indentations caused by the screws and/or nails, imperfections in the wallboard panels, as well as other materials (e.g., corner beads) used to form the wall.
The prior art discloses a number of joint compound compositions (sometimes referred to by applicators as "mud") that can be used to conceal the joints between adjacent sheets of wallboard. Such compositions can also be applied to other defects in the wallboard, for example the defects or depressions caused by screws or nails used to hold the wallboard in place.
In order to conceal the joints between wallboard panels, the joint compound is typically applied in several layers or coats. A first coat of joint compound is placed in the joint between the wallboards with a wallboard knife, or trowel. The first coat is primarily for the purpose of filling the space between the adjacent wallboards. Joint tape (for example, made of paper) can then be embedded into the first coat of joint compound. It is conventional to apply the joint compound in several, e.g., three, coats or layers in order to obtain a smooth finish, and each coat of joint compound must be allowed to dry prior to the placement of the next coat of joint compound. (If the prior coat is not allowed to dry, problems such as excess shrinkage and/or cracking can occur.) Once dry, the treated area is sanded before applying the next finishing coat of joint compound. Joint compound may be used for other purposes, for example, to repair imperfections in various building materials and for adding texture to walls and ceilings.
There are many different types of joint compound. Joint compounds can be supplied in a dry powder form or in a mill mixed ready-mix form. They can also be of a "drying" type or a "setting" type.
The joint compound can be supplied in the form of a dry powder, to which an amount of water is added at the work site by the applicators to give the joint compound a suitable consistency. Other joint compounds, often referred to as "ready-mix" or "ready-mixed" joint compounds, are pre-mixed with water and other additives during manufacturing the product. These joint compounds are generally packaged and sold in a corrugated box or plastic pail in a form that is suitable for use with little or no addition of water at the job site. Ready-mixed joint compounds have several advantages such as providing a consistent product formulation, reducing or eliminating the time necessary to mix the joint compound with water on the job site, and eliminating the need to have water available at the job site.
Among the prior art joint compound compositions, it is known to generally use a filler (e.g., calcium carbonate, calcium sulfate hemihydrate, or calcium sulfate dihydrate) thickener, preservative and a binder, as well as various other additives to produce a joint compound. See U.S. Pat. No. 5,653,797, the disclosure of which is hereby incorporated herein by reference. General ranges of ingredients used in an all purpose, conventional-weight joint compound include the ingredients shown in Table I below.
TABLE I CONVENTIONAL WEIGHT JOINT COMPOUND Weight Ingredient Percentage Water 20-37 Preservatives 0.02-1.0 Calcium Carbonate 10-100 Mica (Filler) 0.5-5.0 Attagel Clay (non-leveling agent) 0.5-5.0 Cellulose Thickener 0.12-1.0 Latex (Binder) 1.0-4.0 Accelerator 0.01-2.0
Many joint compounds are of the "drying," or "setting report," type. A large portion of a drying-type joint compound, comprises calcium carbonate (CaCO.sub.3) and/or calcium sulfate dihydrate (CaSO.sub.4.multidot.2H.sub.2 O). Prior to use (generally during manufacturing), these components and a binder (along with several other known ingredients) are mixed for a specific time with water. After application, when the water dries (i.e., evaporates), a dry, relatively hard cementitious material is left behind. The calcium sulfate dihydrate and calcium carbonate may comprise a substantial portion of what is sometimes referred to as the filler component.
In order to avoid various disadvantages of the drying type of joint compound, compounds of the "setting type" have been developed. A setting-type joint compound includes calcium sulfate hemihydrate (CaSO.sub.4.multidot.1/2H.sub.2 O, also referred to as calcined gypsum). (See U.S. Pat. No. 5,653,797, previously incorporated herein by reference.) In order to produce calcined gypsum, one converts calcium sulfate dihydrate from raw gypsum to the hemihydrate state through a known process called calcination. This process removes one and one-half molecules of water from the calcium sulfate dihydrate gypsum. The hemihydrate form of calcium sulfate is substantially more soluble in water than the dihydrate form of calcium sulfate. The calcium sulfate hemihydrate may comprise a substantial portion of what is sometimes referred to as the filler component.
During use in a setting joint compound, the calcium sulfate hemihydrate is rehydrated to the dihydrate state. This rehydration process normally takes place over a fairly short period of time. Accordingly, it has been extremely difficult to produce a joint compound comprising hemihydrate gypsum because the product would set in its storage container. Thus, joint compounds of the setting type have generally had to be supplied in the powder form. Setting joint compounds have the advantage of having generally quicker finishing times (setting time) than drying joint compounds. This is advantageous for the reasons stated above. Further, because setting joint compounds form a crystalline network upon setting (as opposed to merely drying), these compounds typically provide a stronger, more durable bond between adjacent wallboard sheets than do drying type joint compounds. However, because these compounds provide a stronger bond between adjacent wallboard sheets, the joint compound is often harder to sand to a smooth finish.
Although the industry has several types of joint compounds, joint compounds (particularly ready-mixed materials) are all generally manufactured by the same type of process, a batch process. However, using a batch process to manufacture joint compound creates several disadvantages. First, use of a batch process will limit the maximum production rate due to the longer mixing time and the need to complete one batch before beginning the next. Second, residence time in apparatus used for batch processes may be high, which typically causes relatively long production times. Third, in batch-mixing process, material build-up typically occurs in mixers used for batch operations, often resulting in long down times for cleanup, maintenance, and contaminations.
As demand for joint compound in the industry increases, it would be desirable to produce joint compound faster and more efficiently. It would also be desirable to provide a process for producing a joint compound product that does not vary from one batch to another. Moreover, it would be advantageous to provide a more effective process for producing joint compound products that reduces or eliminates the need for certain ingredients, thereby reducing the cost and complexity of the manufacturing process.